I found some faint hints that Clang has -mdisable-tail-calls (see disable-tail-calls.c | searchcode <https://searchcode.com/file/307703938/src/llvm-project/clang/test/CodeGen/disable-tail-calls.c/> ) , so I figured I may try -menable-tail-calls. Not sure if that did anything, but I ended up with a very small difference (6 bytes) in file sizes for my FFmpeg build (42789880 vs 42789886 bytes). Not sure where that comes from. That being said, the aforementioned ~1% performance difference could indeed be noise. I'll do some more tests to see what's going on. I may have to disassemble the wasm binary to figure out if there really is anything as a result of the -menable-tail-calls flag.
On Wednesday, November 24, 2021 at 4:49:26 PM UTC+10 [email protected] wrote: > Hmm, I tried finding documentation on -menable-tail-call or an > implementation of it but couldn't, so then I tried using it to see if it > did anything. I built the in-tree sqlite benchmark at -O2 with no extra > flags, with -mtail-call, and with -menable-tail-call. The version built > with -mtail-call contained 53 instances of `return_call` or > `return_call_indirect` instructions, but the version with no flags and the > version with -menable-tail-call were identical. I also confirmed that > Emscripten does not warn or error out on unknown flags passed at link time, > so I can only conclude that -menable-tail-call is not a real flag. The > performance difference you saw must have been noise. > > On Wed, Nov 24, 2021 at 12:00 AM Soeren Balko <[email protected]> > wrote: > >> Thanks Thomas - haven't run into any issues. There wasn't any measurable >> performance uplift when using -mtail-call though, which is probably >> because my workload (FFmpeg) may not use it much. >> >> That being said, I previously played around with the -menable-tail-call >> linker flag. And unlike -mtail-call, it does two things: firstly, it >> does produce a small (~1%), but consistent performance uplift. At the same >> time, it doesn't seem to make use of the new WebAssembly opcodes (didn't >> require me to enable those in V8). Not sure what -menable-tail-call >> actually does, but it seems different from -mtail-call. Also, it cannot >> be used as a compiler flag, ie. only the linker will accept it. >> >> Any ideas? >> >> Soeren >> >> On Wednesday, November 24, 2021 at 10:57:46 AM UTC+10 [email protected] >> wrote: >> >>> Hi Soeren, >>> >>> The compiler flag -mtail-call is what you want. This support has been in >>> upstream LLVM for a while, but it hasn't been used very much, so if you run >>> into bugs with it, please let us know! >>> >>> Best, >>> >>> Thomas >>> >>> On Tue, Nov 23, 2021 at 6:14 PM Soeren Balko <[email protected]> >>> wrote: >>> >>>> I was wondering about the state of WebAssembly tail call support in >>>> Emscripten. It's available as an experimental V8 feature in Chrome and >>>> will >>>> hopefully be mainstreamed in the not-too distant future. >>>> >>>> Couldn't find much information on how to build with tail call support >>>> in Emscripten, but I suspect that adding the -menable-tail-call linker >>>> flag does the trick? Or is there more to it? >>>> >>>> @Alon - can you please confirm? >>>> >>>> Thank you! >>>> Soeren >>>> >>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "emscripten-discuss" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to [email protected]. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/emscripten-discuss/bdcc8072-b7a0-454c-bd05-68b0ae780c77n%40googlegroups.com >>>> >>>> <https://groups.google.com/d/msgid/emscripten-discuss/bdcc8072-b7a0-454c-bd05-68b0ae780c77n%40googlegroups.com?utm_medium=email&utm_source=footer> >>>> . >>>> >>> -- >> You received this message because you are subscribed to the Google Groups >> "emscripten-discuss" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> > To view this discussion on the web visit >> https://groups.google.com/d/msgid/emscripten-discuss/f697836d-10ef-41a0-85bb-565698302427n%40googlegroups.com >> >> <https://groups.google.com/d/msgid/emscripten-discuss/f697836d-10ef-41a0-85bb-565698302427n%40googlegroups.com?utm_medium=email&utm_source=footer> >> . >> > -- You received this message because you are subscribed to the Google Groups "emscripten-discuss" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/emscripten-discuss/4552e92c-983b-4dff-a38a-ce34c7c50ca6n%40googlegroups.com.
